1. Field of the Invention
The present invention relates to a method of controlling a document-printing arrangement and a document-printing arrangement which can be controlled by this method and has a paper transport device for a continuous paper web, in whose paper path there are arranged a pair of drive rollers driven by a motor and a print head which cooperates with a print backing means.
2. Description of the Related Art
International patent application WO 94/21465 A1 has disclosed a document-printing arrangement in which a recording medium--a continuous paper web or else a single sheet--is transported on a paper path by a transport device arranged upstream of a printing station. The transport device is formed by a pair of transport rollers driven by an electric motor. Arranged downstream of the printing station in the paper path is a cutting device for cutting a document off the continuous paper web. Arranged in the paper path between the printing station and the cutting device is a sensor which is suitable for determining the position of the leading edge of the paper web.
A similar document-printing arrangement is also described in the international patent application WO 94/22117 A1. In this application a recording medium--which may be a continuous paper web or else a single sheet--is transported by two transport devices which are arranged one behind the other in the paper transport direction and are driven at the same time. One of the transport devices, formed by a pair of transport rollers, is arranged upstream of a printing station. Although in theory both transport devices have the same transport speed, these may be slightly different because of unavoidable mechanical tolerances. If the pair of transport rollers arranged upstream of the printing station transports more slowly than the other transport device, this manifests itself in a distorted printed image. In the event of stiffness of the recording media, too and thus in the case of an otherwise advantageous stepping motor as the transport roller drive, stepping losses may occur, so that a controlled paper advance is no longer possible.
A further problem may arise if the delivery directions of the two transport devices differ from one another because of circumferential tolerances over the length of the transport rolls. This leads to the continuous paper web running crookedly, which results, at least, in a distorted printed image.
In the document-printing arrangement disclosed by the published international patent application WO 94/21465, the recording medium, in addition to being transported by the abovementioned paper transport device, is also transported by a second transport device, which is arranged downstream of the printing station in the paper transport direction and is driven at the same time as the first paper transport device. The above-mentioned problems therefore also occur for the document-printing arrangement disclosed here.